X-rays can interact with matter through various processes. The most common interactions in diagnostic radiology are the photoelectric effect, Compton scattering, and coherent scattering. In the photoelectric effect, an incoming x-ray photon ejects an inner shell electron, leaving behind characteristic radiation. Compton scattering involves x-ray photon deflection by a free electron, producing scatter radiation. Coherent scattering changes a photon's direction but not its energy. These interactions have different probabilities depending on the photon energy and the electron binding energy.
2. X-ray Interactions with Matter
For understanding the interactions of ionizing radiation and
matter we must review a few points about atomic physics.
The atom consists of a central nucleus and orbital
electrons.
The positively charged nucleus exerts an electric force of
attraction and holds the negatively charged electrons in
specific orbits or shells. The innermost shell is called the
K shell and more the peripheral shells are named
consecutively L, M, N, and so forth.
They have a limited electron capacity.
The K shell can hold 2 electrons and L shell has capacity
of 8 electrons.
3. Basic structure of an ATOM :
PROTON ( +ve charge )
An atom is made up of NUCLEUS
NEUTRON ( neutral )
ORBITAL ELECTRONS ( -ve charge )
ORBITS / SHELLS ( K, L, M, N etc. )
4. Each shell has specific binding energy. The closer the
shell is to nucleus, the tighter the electrons are ‘bound’ to
the nucleus.
The electrons in the outer most shell are loosely bound.
They are essentially free and are called ‘free’ or ‘valence’
electrons.
5. There are 5 basic ways that an x-ray photon can interact
with matter. These are:
1. Photoelectric effect
2. Coherent or unmodified scattering
3. Compton interaction with modified scattering
4. Pair production
5. Photodisintegration
6. 1. Photoelectric effect
The photoelectric effect always yields three end products:
1) Characteristic radiation
2) A negative ion ( the photoelectron)
3) A positive ion ( an atom deficient with one electron)
7.
8. Characteristic radiation generated by the photoelectric effect
is exactly the same as production of x-rays
The only difference in the method used to eject the inner
shell electron.
In x ray tube a high speed electron ejects the bound
electron.
While in photoelectric effect an X ray photon does the trick.
In both cases the atom is left with an excess of energy =
the binding energy of an ejected electron.
It is usually referred to as ‘Secondary Radiation’ and is to be
differentiated from scatter radiation.
CHARACTERISTIC RADIATION
9. How does this happen ?
After the electron has been ejected, the atom is left with a
void in the K shell & an excess of energy equivalent to the
binding energy.
This state of the atom is highly unstable & to achieve a
low energy stable state ( as all physical systems seek the
lowest possible energy state ) an electron immediately
drops in to fill the void.
As the electron drops into the K shell, it gives up its
excess energy in the form of an x-ray photon. The amount
of energy released is characteristic of each element &
hence the radiation produced is called Characteristic
radiation.
10. Photoelectric Effect
This type of interaction is most likely to occur when the energy
(hv) of an incident/ incoming photon with slightly greater energy
than the binding energy of the electrons in one of the inner shells.
The incident photon looses all its energy on entering an atom
being absorbed in this process.
Immediately, the atom responds by ejecting an electron, usually
from K or L shell, leaving a hole in that shell.
Now the atom is ionized positively and in an excited state.
Electron from higher energy level fills the hole in the K shell, a
‘characteristic x-ray photon’ is being emitted.
Note: that the energy of the incident photon ultimately went to free
the electron from its shell and set it motion as ‘photoelectron’
11. Summary: The energy of the incoming photon in the photoelectric
interaction involving the K shell has the following fate:
a) The photon enters atom and completely disappears.
b) A K-shell electron is ejected, leaving a hole.
c) Atom has excess energy – is in a excited state.
d) A part of photon’s energy was used to liberate electron and the
rest to give it kinetic energy; ejected electron is a photoelectron.
e) Hole in K shell is filled by electron transition from a shell father
out, accompanied by emission of a characteristic x-ray photon.
f) Holes in successive shells are filled by electron transitions from
shells still farther out, each transition accompanied by a
corresponding characteristic x-ray photon.
g) Sum of the energies of all the characteristic photons equals to
binding energy of shell from which the photoelectron originated,
in this case, the K shell.
12. Probability of occurrence:
3 simple rules govern the probability of occurrence
1) The incident photon must have sufficient energy to overcome
the binding energy of the electron.
2) A photoelectric reaction is most likely to occur when the photon
energy and electron binding energy are nearly the same.
3) The tighter an electron is bound in its orbit, the most likely it is to
be involved in the photoelectric reaction.
Application to diagnostic radiology:
Advantage:
It produces radiological images of excellent quality.
Does not produce scatter radiation.
It enhances natural tissue contrast (as some tissues absorb more
x-rays than other tissues.
Disadvantage:
Patients receive more radiation.
All the energy of incident photon is absorbed by the patient.
13. Coherent or unmodified
scattering
Radiation undergoes a change in direction without change in
wavelength, thus sometimes it is called as “ unmodified
scattering”
There are two types of coherent scattering:
Both the types are described in terms of wave-particle
Interaction and therefore also called as ‘Classical
scattering’
i. Thomson scattering: Single electron involved in the
interaction.
ii. Rayleigh scattering: there is Co-operative interaction of
all the electrons.
14.
15. What happens in coherent scattering ?
Low energy radiation encounters electrons
Electrons are set into vibration
Vibrating electron, emits radiation.
Atom returns to its undisturbed state
16. • No energy is transferred and no ionization occurs.
• Its only effect is to change direction of incident
radiation.
• It occurs less than 5% and is not important in
diagnostic radiology. It produces scattered
radiation but of negligible quantity.
17. Compton interaction
The Compton effect occurs when an incident x-ray photon with
relatively high energy strikes a free outer shell electron, ejecting
it from its orbit.
The photon is deflected by the electron so that it travels in new
direction as scatter radiation.
The reaction produces an ion pair
A +ve atom
A –ve electron ( recoil electron )
18. Almost all the scatter radiation that we encounter in diagnostic radiology
comes from Compton Scattering.
Energy of photon distributed in two ways:
Part of it goes to recoil electron as Kinetic energy.
And the rest is retained by the deflected photon.
Two factors determine the amount of energy the photon transmits:
The initial energy of the photon.
Its angle of deflection.
1.Initial energy :- Higher the energy more difficult to deflect.
High energy : Travel straight retaining most of the energy.
Low energy : Most scatter back at angle of 180º
2. Angle of deflection :- Greater the angle, lesser the energy
transmitted. With a direct hit, maximum energy is transferred to the
recoil electron. The photon retains some energy & deflects back
along its original path at an angle of 180º.
19. The formula for calculating the change in wavelength of a scattered photon
is :
Δλ = 0.024 ( 1 – cos θ ) ,
where Δλ = change in wavelength
θ = angle of photon deflection
20. Disadvantages of Compton reaction :
Scatter radiation : Almost all the scatter radiation that we
encounter in diagnostic Radiology comes from Compton scattering. In
the diagnostic energy range, the photon retains most of its original
energy. This creates a serious problem, because photons that are
scattered at narrow angles have an excellent chance of reaching an x-
ray film & producing fog.
Exceedingly difficult to remove –
• cannot be removed by filters; because they are too energetic
• cannot be removed by grids; because of narrow angles of deflection.
It is also a major safety hazard. Even after 90˚ deflection most of its original
energy is retained.
Scatter radiation as energetic as the primary radiation.
Safety hazard for the radiologist, personnel and the patient.
21. Pair Production
What happens in Pair production ?
A high energy photon interacts with the nucleus of an atom.
The photon disappears & its energy is converted into matter in
the form of two particles
An electron
A positron (particle with same mass as electron, but with
+ve charge.)
Mass of one electron is 0.51 MeV.
2 electron masses are produced.
So the interaction cannot take place with photon energy less
than 1.02 MeV.
It has no importance in diagnostic radiology.
22. Annihilation reaction:
The Positron, as it comes to rest, combines with a negative electron-it
disappears giving rise to two photons with an energy of 0.51 Mev, moving in
opposite direction.
This is annihilation reaction.
23. Photodisintegration
A photon with extremely high energy ( 7-15 MeV), interacts
directly with the nucleus of an atom. It does not occur with
energies less than 7 Mev,
which may eject a neutron, proton or on rare occasions even an
alpha particle.
What happens in Photodisintegration ?
A high energy photon encounters the nucleus of an atom.
Part of the nucleus which may be a neutron, a proton, an alpha
particle or a cluster of particles, is ejected.
It has no diagnostic importance. As we rarely use radiation>150
KeV in diagnostic radiology.
24. To Note:
The photoelectric effect accounts for 75% of
interaction.
Compton scattering for 20%, and
Coherent scattering for 5%,
the total 100%.
26. Ans. The photoelectric effect
always yields three end products:
Characteristic radiation
A negative ion ( the
photoelectron)
A positive ion ( an atom deficient
with one electron)
28. Ans. Annihilation reaction:
The Positron, as it comes to rest,
combines with a negative
electron- it disappears giving rise
to two photons with an energy of
0.51 Mev, moving in opposite
direction.
This is annihilation reaction.
